In recent years, the spread of portable electronic devices such as digital cameras, smart phones and tablet devices has been remarkable. This has been accompanied by a large growth in demand for secondary batteries that can be charged and repeatedly used, as well as an increasing desire for higher capacity and higher energy density in such batteries.
Of these, lithium-based secondary batteries with a high cell voltage of 3V or more and a large energy density per unit weight have attracted particular attention and are today the subject of vigorous development efforts.
An electrolyte solution obtained by dissolving an ion-conductive salt such as LiBF4 or LiPF6 in an aprotic organic solvent is generally used in these lithium-based secondary batteries.
Most lithium-based secondary batteries are designed so as to charge and discharge within the voltage range between a fully charged cell voltage of 4.2 V and an end-of discharge voltage of about 2.7 V. However, in high-voltage secondary batteries having a voltage of 4 V or more, the organic solvent and electrode active materials used are sometimes exposed to high voltages and electrically decompose.
Various improvements in the materials making up lithium-based secondary batteries have been attempted to date in order to solve such problems. One such technology involves the use of an ionic liquid as a component of a liquid electrolyte.
For example, Patent Document 1 discloses a secondary power supply which uses an electrolyte that includes a lithium salt, an ionic liquid and an organic solvent. More specifically, this publication discloses a secondary power supply in which 1-ethyl-3-methylimidazolium tetrafluoroborate (abbreviated below as EMIBF4) is used as the ionic liquid and the molar ratio thereof with respect to lithium salt is set in a specific range. However, a drawback of EMIBF4 is that, although it has a relatively low viscosity, because the withstand voltage is low, the charging voltage cannot be raised, making use in a high voltage region impossible.
Patent Document 2 discloses a liquid electrolyte which includes both an ionic liquid containing an alicyclic ammonium cation having a specific alkoxyalkyl group on a nitrogen atom, and an ion-conductive salt that is a solid at room temperature. Specifically, a liquid electrolyte containing N-methoxyethyl-N-methylpyrrolidinium tetrafluoroborate (MEMPBF4) and the like as the ionic liquid, and an electrical double-layer capacitor that uses the same are disclosed. However, MEMPBF4 does not have a sufficient ability to dissolve lithium salts, and so increasing the lithium salt concentration is difficult. In addition, because it has a high viscosity, when the ionic liquid alone is used as a liquid electrolyte, the internal resistance of the battery rises.
Patent Document 3 discloses a lithium-ion capacitor which uses a liquid electrolyte that includes both a lithium salt and an ionic liquid, wherein the lithium salt and the ionic liquid have the same anion. Specifically, a capacitor is disclosed in which a compound such as N-methyl-N-butylpyrrolidinium bis(fluorosulfonyl)amide (MBPYFSA) is used as the ionic liquid and lithium bis(fluorosulfonyl)amide is used as the lithium salt. However, in the case of a compound such as MBPYFSA, because the ionic liquid itself has a higher viscosity than EMIBF4, adding a lithium salt further increases the viscosity, lowering the charge-discharge characteristics when such an electrolyte is included in a lithium-ion battery.
Patent Document 4 discloses various bis(fluorosulfonyl)amide anion-containing ionic liquids and methods for their synthesis, and also discloses that these ionic liquids can be used as electrolyte materials in secondary batteries and the like. Although N-methoxyethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide (MEMP.FSA) is disclosed here as a specific ionic liquid, no mention is made of the characteristics of secondary batteries in which this is used.
Because there are cases in which, depending on the service environment and conditions, electrical energy storage devices such as secondary batteries are exposed to elevated temperatures, stability under such elevated temperatures is also demanded. However, none of the foregoing literature makes any mention of findings relating to the performance of secondary batteries in high-temperature environments.